1. Field of the Invention
This invention relates to an improved process for coating a conductor with fluoropolymer insulation and to a fluoropolymer therefor.
2. Description of Related Art
U.S. Pat. No. 5,703,185 discloses an improved fluoropolymer, wherein the improvement enables the fluoropolymer to be extruded and the molten polymer to be drawn down (melt-draw down) to form insulation on a conductor at higher line speeds. Extrusion/melt draw-down to form insulated conductor is shown on pp. 17-19 of the TEFLON®/TEFZEL® Melt extrusion Guide, published by the DuPont Company in March, 1993. The extrusion conditions in Example 10 of '185 include the line speed, the draw down ratio for the melt draw-down step (97:1), the melt temperature increasing from 757 to 774° F. (403 to 412° C.) as the line speed is increased, and the copolymer having a melt viscosity (MV) of 2.37×103 Pa.s at standard conditions. This melt viscosity is calculated from the melt flow rate in g/10 min (MFR) of the copolymer through a specified orifice under a specified weight in accordance with ASTM D1238-52T modified as described in U.S. Pat. No. 4,380,618 (calculation: MV=53150/grams of copolymer flowing through the orifice in 10 min). Thus, in Example 10 it is disclosed that the tetrafluoroethylene/hexafluoropropylene/perfluoro(ethyl vinyl ether) (TFE/HFP/PEVE) copolymer can be extruded and drawn down onto the conductor at line speeds of 1500 to 3000 ft/min (456 to 914 m/min) to form insulation that has low incidence of spark failures, i.e. ranging from zero at 13 km coated length of conductor to 4 for 14.3 km coated length of conductor. In contrast, the corresponding TFE/HFP copolymer wherein the additional monomer is perfluoro(propyl vinyl ether) (PPVE) could not form insulation of comparable low failure incidence at line speeds above 1900 ft/min (579 m/min). In commercial practice, line speeds in excess of 2250 ft/min (686 m/min) are difficult to control, whereby line speeds in the range of 1750 to 2250 ft/min (533 to 686 m/min) are considered highly desirable.
The problem has arisen that when the fluoropolymer is supplied to a multiplicity of insulated conductor manufacturers, the extrusion/melt draw-down process is carried out under a variety of conditions, resulting in the fluoropolymer performing better for some manufacturers than for others, and for each manufacturer, better on some days than on others. The UL 444 industry standard for spark failures is no more than 15 spark failures per 45,000 ft (13.7 km) of coated conductor. A spark failure indicates a fault in the insulation. Industry prefers that no more than 10 spark failures be present/13.7 km of insulated conductor to insure acceptable insulated conductor. An additional quality criterion desired by the industry is that for the same length of coated conductor, the insulation should have no more than 2 lumps/13.7 km. Lumps in the insulation interfere with the ultimate use of the insulated conductor; e.g. twisting together to form twisted pair conductors, pulling the insulated conductor through narrow openings.
The manufacturer is faced with the question of how to consistently maintain high productivity of insulated conductor of the desired quality. The manufacturer can also change certain process conditions. These are: a) polymer melt temperature, b) line speed, c) tooling, i.e. the sizing of the guide tip for the conductor and the diameter of the extrusion die, which determines the annular gap for the molten resin between the O.D. (outer diameter) of the die and the O.D. of the guide tip), and d) cone length (distance between the die through the tubing of molten polymer is extruded and the point at which the drawn molten tube (in the form of a cone) contacts the conductor).
The tooling determines the draw-down ratio of the extrusion/melt draw-down step. Draw-down ratio (DDR) is the ratio of the cross-sectional area of annular gap through which the molten resin is extruded and the cross-sectional area of the resultant insulation. A wide range of draw-down ratios are used in the industry, typically between 60 to 120:1. The manufacturer can change the tooling in order to change the DDR, but this requires extruder purging and cool-down, the tooling change, then heat-up and recharging with fluoropolymer. This is time-consuming and economically infeasible and moreover, it is not clear whether the DDR should be increased or decreased in order to solve insulation quality problems at high line speed. Even if a change in DDR produces success, this may only be temporary, since the DDR changes with different gauges of conductor coated with the fluoropolymer insulation and with the use of a given gauge because of gauge variations along the conductor length.
As in the case of changing DDR, the possibility of changing temperatures and cone length are “hit and miss”, i.e. by guesswork. Cone length is changed by changing the vacuum that draws the tubing of molten polymer down onto the conductor, thus forming the cone in the transition between extrusion and conductor contact. Temperature changes are typically carried out by changing the temperature profile along the length of the extruder. Success by temperature changes tends to be temporary, because some other change in the process or in the polymer adversely affects the quality of the insulation. For this reason, most manufacturers simply monitor the temperature profile along the length of the extruder, but not the melt temperature at the time of extrusion.
The manufacturer of the insulated conductor is also faced with the variability introduced into the fluoropolymer by adding pigment to the fluoropolymer prior to extrusion/melt draw-down, with pigment addition varying in amount and type to obtain the insulation color desired. The manufacturer must consider whether an insulation quality problem arises from the addition of the pigment to the fluoropolymer. The most common pigments used can be reasonably represented by the use in experiments of both white and orange.
Faced with the onset of quality (spark and/or lump) problems, and at least the uncertainty of changing DDR, operating temperatures, and cone length, the manufacturer typically reduces line speed until the desired quality insulated conductor is reached, resulting in a loss of productivity.
From the standpoint of the fluoropolymer manufacturer, the challenge is whether there is improvement possible in the fluoropolymer that the would accommodate the unavoidable differences among customers and among extruders and their operators and permit high quality insulated conductor to be made at high speeds.